Although not part of the present invention, a basic understanding of the Vermont Spinal Fixator is important to the understanding of the present invention. FIG. 1 illustrates one half of a Vermont Spinal Fixator device 10 in place on a spine, the same components are also used on the opposite side of the spine and are not shown. The fixator device is designed to rigidly fix together two spinal vertebrae surrounding a fractured vertebrae and, thus, fuse the spine around the fractured vertebrae. The Vermont Spinal Fixator is disclosed in detail in Krag et al., An Internal Fixator for Posterior Application to Short Segments of the Thoracic, Lumbar, or Lumbosacral Spine, Clinical Orthopaedics and Related Research, 203: 75-98 (February 1986).
In order to implant the fixator device 10, holes are drilled in the appropriate vertebrae through the pedicle on either side of each vertebrae. After the holes are drilled, pedicle screws 12 are screwed into place using a shaft handle 14 which is attached to flats 16 provided on the top of each screw 12. The shaft handles 14 are best seen in FIG. 2. Once the pedicle screws 12 are in place, each one has an articulating clamp 18 attached to it by means of a clamp bolt 20.
Clamp bolt 20 is placed through clamp 18 and loosely threaded into the head of pedicle screw 12. The clamp bolts 20 are left loose until realignment of the vertebrae by the reduction frame has been completed. Shaft handles 14 remain attached to the tops of the pedicle screws 12. The shaft handles may be provided with removable grips which are not shown in FIG. 2.
As a result of various spinal disorders of the type which the fixator device 10 is intended to remedy, such as trauma, one vertebra is displaced to an abnormal position relative to an adjacent vertebra. For this reason the surgeon must manipulate the vertebrae back into normal alignment before the clamp bolts 20 are finally tightened and the spine is rigidly fixed in position.
In the past, in order to place the spine in proper alignment, the surgeon would either move the lower half of the patient's body with respect to the upper half, or would grasp the shaft handles 14 and use them as separate levers to manipulate the spine.
These procedures have a number of disadvantages. First, they not allow for fine control of the alignment and force applied, and also require the surgeon to hold the spine exactly in alignment while the clamp bolts are tightened. Second, when the handles are grasped and pushed together manually to produce extension, a compressive force is also produced. This compressive force is difficult to prevent manually and can cause bone fragments to be pushed posteriorly against the spinal cord. Third, while flexion and extension of the spine are generally possible with this technique, distraction (the in-line spreading apart of the spine) and compression are difficult, if not impossible, to achieve with accuracy and control. The difficulty arises due to the fact that the shaft handles act as levers on the spine, thereby tending to cause rotation of the vertebrae, which may not be desired. Fourth, there is a danger that excessive force may be applied to either the right or left pedicle screw during the application of force for accomplishment of realignment. This danger is present because of the absence of a rigid linkage between the right and left pedicle screws to provide an automatic balancing of forces.
There are a number of implants and reduction frame type devices in the prior art. However, none of them overcome the above disadvantages, at least without creating other disadvantages. For example, U.S. Pat. No. 3,865,105 to Lode discloses a device for exerting force on and fixing the spinal column. The Lode device appears to be designed primarily for the correction of scoliosis. The arrangement of this device renders it impractical for use in producing realignment of vertebrae affected by fractures and dislocations. The amount of control provided is limited as it is much less of a factor in applications such as straightening an unfractured spine contemplated by the Lode device. Also, the three point attachment directly to the transverse process or spinous process would obstruct the surgical area such that a spinal fixator device being implanted could not be easily accessed.
U.S. Pat. Nos. 4,433,677; 4,658,809 and 4,854,304 all show spinal fixation devices which are adjustable for distraction and compression. These all exhibit the primary disadvantage of not allowing flexion-extension adjustment which is often necessary for alignment of various spinal disorders including fractures and dislocations. These devices also generally employ turnbuckle type adjustments which are difficult to use under surgical conditions and do not offer significant mechanical advantage.